The invention relates to a system having an electrically excited machine, which is controlled and is supplied with electrical energy by means of a controllable first energy store, and to a method for operating the system of the invention.
The trend is that in the future electronic systems which combine new energy store technologies with electrical drive technology will be used increasingly both in stationary applications, for instance wind power plants, and in vehicles, such as hybrid or electric vehicles. In conventional applications, as shown in FIG. 1 for example, an electric machine 101 which is in the form of a polyphase machine, for example, is controlled by means of a converter in the form of a pulse-controlled inverter 102. A characteristic of systems of this type is a so-called DC voltage intermediate circuit 103 via which an energy store 104, generally a traction battery, is connected to the DC voltage side of the pulse-controlled inverter 102. In order to be able to meet the requirements placed on power and energy for a respective application, a plurality of battery cells 105 are connected in series. Since the current provided by an energy store 104 of this type must flow through all battery cells 105 and a battery cell 105 can only conduct a limited current, battery cells are often additionally connected in parallel to increase the maximum current.
The series connection of a plurality of battery cells entails the problem, in addition to a high total voltage, that the entire energy store fails when a single battery cell fails because then no battery current can flow any more. Such a failure of the energy store can lead to a failure of the entire system. In the case of a vehicle, a failure of the drive battery can cause the vehicle to “break down”. In other applications, for instance the rotor blade adjustment of wind power plants, hazardous situations may even arise in the event of unfavorable boundary conditions, for instance a strong wind. Therefore, a high degree of reliability of the energy store is always desired, where “reliability” is intended to mean the capacity of a system to operate fault-free for a predetermined time.
In the earlier applications DE 102010027857.2 and DE 102010027861.0, batteries having a plurality of battery module strings have been described which can be connected directly to an electric machine. In this case the battery module strings have a plurality of series-connected battery modules, wherein each battery module has at least one battery cell and an associated controllable coupling unit, which makes it possible, depending on control signals, to interrupt the respective battery module string or to bypass the respectively associated at least one battery cell or to connect the respectively associated at least one battery cell into the respective battery module string. By suitably actuating the coupling units, for example with the aid of pulse-width modulation, it is also possible for suitable phase signals for controlling the electric machine to be provided with the result that a separate pulse-controlled inverter is not required. The pulse-controlled inverter required for controlling the electric machine is therefore integrated in the battery, so to speak. For the purposes of the disclosure, these two earlier applications are incorporated in full in the present application.
In contrast to conventional systems, a constant DC voltage which can be used for example to supply an exciter winding of a separately excited electric machine is not available at the output of the battery system.